1. Field of the Invention
This invention relates to a method of and apparatus for comparing relative surface reflectivity.
2. Description of the Prior Art and Technical Problems
In the handling of sheets having a coated surface, it is necessary to determine the orientation of the coated sheet surface so that the sheet is properly handled in subsequent operations. There are available in the prior art, e.g., U.S. Pat. Nos. 2,963,938; 3,666,360 and 3,792,930 techniques for inspecting sheets, e.g. glass sheets for internal and external defects. Although the techniques taught in the above-mentioned patents are acceptable for their intended purposes, they are not designed to distinguish between coated and uncoated sheet surfaces. Also in the prior art e.g., U.S. Pat. Nos. 3,572,951; 4,035,085 and 4,072,426 there are taught techniques for inspecting reflective coatings on glass sheet surfaces. Although these systems are acceptable for their intended purposes, there are no teachings therein for adopting these techniques for distinguishing between a coated and an uncoated surface of a glass sheet to determine sheet orientation.
U.S. Pat. No. 2,251,613 teaches a system for determining smoothness of a sheet surface. In general, a light source has its light beams incident on an ink mark on a sheet surface. The light beams of another light source are incident on the opposed surface. The intensity of the reflected beams from each surface are compared to determine the comparative smoothness of the sheet surface. The drawbacks of the system taught in U.S. Pat. No. 2,251,613 are the additional steps required to impose and remove the ink mark and the risk of damaging the surface while the mark is being imposed and/or removed.
From the above it can be appreciated that it would be advantageous to provide a system for comparing and/or distinguishing between reflective surfaces of a sheet.